The present invention relates generally to a method and apparatus for cutting discrete quantities of molten plastic material from a supply of the molten plastic material for subsequent compression molding of the discrete quantities of material. More particularly, the invention relates to a cutter which rotates relative to a molten plastic delivery nozzle to cut a pellet of molten plastic from the nozzle and then carries and ejects the pellet of molten plastic into a cavity for compression molding articles therefrom.
U.S. Pat. No. 4,277,431, to Peller, hereby incorporated by reference, discloses an apparatus for cutting discrete quantities or pellets of molten plastic material for subsequent placement in respective mold cavities. This apparatus is particularly suited for use in the manufacture of closures by compression molding, including the formation of compression molded closure shells, and the formation of compression molded liners within associated closure shells. U.S. Pat. Nos. 4,343,754 to Wilde et al., and 4,497,765 to Wilde et al., both hereby incorporated by reference, disclose compression molding of threaded, tamper-indicating plastic closures, and compression molding of liners in such closures, for which manufacturing processes the apparatus of the above U.S. Pat. No. 4,277,431 is suited for use.
The apparatus of U.S. Pat. No. 4,277,431 includes a nozzle through which molten plastic material is delivered from an associated extruder or the like, and a rotatably driven cutting blade which is rotated with respect to the nozzle. As plastic is extruded from the nozzle, a discrete quantity or pellet of plastic material is cut during each rotation of the associated cutting blade. Immediately thereafter, the severed plastic pellet is moved from the face of the nozzle by the cutting blade for delivery to a respective mold cavity. The mold cavity may comprise either a female mold die for formation of a closure shell by compression molding, or a closure shell within which the molten plastic is compression molded for formation of a sealing liner.
Notably, the cutter apparatus of the above patent is configured to facilitate separation of each plastic pellet from the cutting blade by creating a slight mechanical interference between the cutting blade and the face of the associated nozzle. Thus, as the cutting blade rotates with respect to the nozzle, the blade is flexed or deflected as it engages the nozzle face and severs the extruded plastic material. As the blade continues to rotate, with the severed plastic material carried on the flexed surface of the cutting blade, the cutting blade disengages the face of the nozzle, thereby rapidly accelerating the pellet to facilitate its separation from the blade and delivery of the pellet to one of the associated cavities. This cutting and subsequent xe2x80x9cflickingxe2x80x9d like action of the cutting blade is sometimes referred to as the xe2x80x9ccut-and-flipxe2x80x9d portion of each cutting cycle.
The above patent contemplates that the disclosed cutting apparatus be mechanically-driven from the associated molding apparatus, thus effecting the desired synchronous operation of the cutter. However, it will be appreciated that increases or decreases in production speed necessarily result in corresponding variation in the xe2x80x9ccut-and-flipxe2x80x9d portion of the cutting cycle, which can create undesirable variability in the speed, direction, rotational velocity, and orientation of the plastic pellet as it is delivered to the associated cavity. This can, in turn, create problems regarding pellet placement, orientation, and an undesirable tendency of the pellet to bounce upon delivery into the associated cavity.
U.S. Pat. No. 5,596,251 describes a cutter apparatus driven by a servo motor, the operation of which is coordinated with an associated rotary carousel on which cavities are successively presented to the cutter apparatus. In order to effect separation of each discrete quantity of plastic material from the cutting blade of the cutter apparatus, the servo motor is operated to create a period of distinct deceleration during each rotary cutting cycle, thereby separating the molten plastic from the surface of the cutting blade.
The present inventors have recognized that it would be desirable to provide a cutter apparatus for cutting molten plastic pellets from a source of molten plastic material and placing the pellets into successive cavities for compression molding which could be effectively operated at a high rate of speed, which reduces the need to replace worn cutter blades, and which reliably operates to produce a high rate of flawlessly molded articles.
The present invention is directed to a plastic pellet cutting system particularly suited for use in compression molding of plastic closure shells, and plastic liners in closure shells. The present invention contemplates a rotating cutting blade operated in conjunction with a molten plastic delivery nozzle to separate plastic pellets from a face of the nozzle. The present system utilizes a cutting blade operated at zero interference (or with slight clearance) with the associated nozzle face, wherein the cutting blade has associated therewith an air driven pellet ejection system. The ejection system uses pressurized air to displace a pellet carried by the blade into a molding cavity.
The present system avoids the need to create mechanical interference between a cutting blade and an associated nozzle face to facilitate separation of each molten plastic pellet from the cutting blade.
In one embodiment, the cutting blade has associated therewith an air activated plunger which reciprocates radially in close proximity to the cutting blade to push a pellet from the blade. The plunger can be assisted by a high velocity stream of air directed at the pellet held on the cutting blade to dislodge the pellet from the cutting blade to deliver the pellet into the molding cavity.
In another embodiment, no plunger is used. The cutting blade has associated therewith an air delivery system which directs a high velocity stream of air at the pellet held on the cutting blade to dislodge the pellet from the cutting blade to deliver the pellet into a molding cavity.
The present system is used in conjunction with a rotary compression molding apparatus, which typically includes a rotating carousel or turret which carries cavities in the form of mold dies or closure shells. The present system effects delivery of discrete quantities of molten plastic material (i.e., plastic pellets) to the series of moving cavities by the provision of an extruder or like apparatus which provides a source of molten plastic material to a nozzle. A cutting blade is driven with respect to the nozzle so that attendant to each rotation of the blade, the blade cuts a discrete quantity or pellet of plastic material as it is being extruded.
The delivery of each molten plastic pellet is effected without significant flexure of the cutting blade, thus obviating the need for mechanical interference between the cutting blade and the nozzle face, as in arrangements heretofore known.
The present invention contemplates a two-piece cutter assembly mounted on a cutter shaft, the cutter shaft being rotated about its axis by a motor. The two-piece cutter assembly comprises a cutter body which is held substantially within a radial bore formed in a distal end portion of the cutter shaft, and a protruding cutter head extending from the cutter body. The cutter shaft includes an internal air passage for pressurized air delivery to the cutter body, and internal cooling channels for passing cooling fluid, such as cooling water, to and from the cutter body for maintaining the cutter body at a desired temperature during operation. The cutter assembly includes a series of air apertures directed toward an internal region of the cutter head for passing pressurized air into the cutter head to dislodge a plastic pellet held thereby, during operation. The air apertures are in flow communication with the internal air passage within the cutter shaft. The air apertures can be located on a plate stationary to the cutter body or on a reciprocating plunger. The internal cooling channels through the cutter shaft are in flow communication with an arcuate area between the cutter body and an inside wall of the radial bore within the cutter shaft.
The cutter shaft is rotated within a rotary union block. The cutter shaft includes a first arcuate channel around its circumference which is in flow communication with the internal air passage. The cutter shaft also includes second and third arcuate channels around its circumference which are in flow communication with the two internal cooling channels, respectively. The rotary union block includes corresponding channels or passages in flow communication with the first, second and third arcuate channels of the cutter shaft, such that pressurized air and cooling water can be sealingly transferred between the rotary union block and the cutter shaft given that the cutter shaft is rotating and the union block is stationary. A precision servo motor drives the cutter shaft via a timing belt and sprocket arrangement.
In operation, the cutter shaft is rotated such that the cutter head sweeps across the nozzle face to cut and carry a molten plastic pellet. At a preselected position in the rotary travel of the cutter head, a stream of pressurized air acts against the plastic pellet either directly and/or via a plunger to dislodge the pellet from the cutter head and into a compression molding cavity.
The preselected rotary position of the cutter head can be defined by the location and circumferential extent of air channels or passages in the rotary union block. Alternatively, the timing of the air delivery of pressurized air into the cutter head can be controlled by a programmable logic controller (PLC).
The invention provides advantages over the prior apparatus. The invention will reduce the probability of broken cutter blades due to fatigue and interference with the nozzle face. The invention will provide more consistent and accurate pellet placement in the compression mold. It is contemplated that the invention will reduce (stringing) of molten plastic during operation, and also decrease a pellet weight standard deviation. It is contemplated that the invention will result in reduced cutter and nozzle face wear and reduced maintenance requirements. Setup time for the apparatus should be decreased and apparatus reliability increased.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.